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<html>
<head>
  <meta charset="UTF-8">
  <title>Document</title>
</head>
<body>
  <script>
    console.log(1);
    setTimeout(function () {
      console.log(2);
    }, 0);
    for (var i = 0, str = ''; i < 900000; i++) {
      str += i;   // 利用字符串拼接运算拖慢执行时间
    }
    console.log(3);
  </script>
</body>
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